Without knowing a bit more of the context of the game it's somewhat hard to come up with reasonable suggestions; what would be a dark secret in a "realistic" domestic game is going to be very different to a dark secret in a superhero game is likely to be different to a dark secret in a medieval game based around warfare etc.

That said there are a couple of obvious tropes to fall back on:

"I'm not who I say I am": The characters "real" history and the history the other characters will know are rather different. This could be an extreme case such as creating a whole new identity by either completely reinventing themselves (or stealing someone elses?) or more minor but still important such as faking a few details... are they from where they say they are? Is their name their real name? Is what they tell people they've been doing for the past few years actually what they've done?"You don't know what I've done": While they may be who they say they are, that doesn't mean they've told everyone their life story. They did something previously and they don't want people to know. Did they kill someone (on purpose or accidentally)? Did they betray a trust? Are they a criminal? Is it something they're ashamed of or something they're not ashamed of but they know people will react (badly?) to?

"If only you could know what I feel": The character is who they say they are and they've done what they've said they've done (and left little out). But no-one knows how they feel as they did it. Perhaps they are seemingly always positive when deep inside their emotions are always dark and depressed. Perhaps they appear cheerful, understanding and forgiving to the outside world... but on the inside they're a seething mass of hatred and rage just looking to break out.

My mistake. I didn't realize it might be hard to help without knowing the context of the story, or my character.

Basically, it's a Silent Hill RP and my character has been pulled into the town due to a "sin" or some deep dark secret. It doesn't necessarily have to be something she was involved in directly.

I ran through most of the "usual deep dark secrets" I could think of. Like murdering someone but it was self defense, having an addiction of some sort, witnessing a crime but not doing anything about it, etc. But none of them seemed to really "fit".